Move more KDC checks to validate_tgs_request()

Move more KDC checks to validate_tgs_request()

Move the following validity checks:

* the INVALID ticket flag check from kdc_process_tgs_req()
* the lineage check from process_tgs_req()
* the user-to-user second ticket client check from process_tgs_req()
* all S4U2Self validity checks from kdc_process_s4u2self_req()
* S4U2Proxy validity checks (but not KDB authorization checks) from
  kdc_process_s4u2proxy_req()

In process_tgs_req(), call validate_tgs_request() after
kdc_process_s4u2self_req() and decrypt_2ndtkt() so that their outputs
can be used as validation inputs.  Add stkt and is_crossrealm locals
for convenience, and remove st_idx.

There are some minor behavior changes:

* For invalid S4U2Self request options, the status string is changed
  from "INVALID AS OPTIONS" to "INVALID S4U2SELF OPTIONS".

* For a header ticket with the INVALID flag, the reply code is changed
  to KRB_AP_ERR_TKT_NYV (as specified in RFC 4120) and the status
  string to "TICKET NOT VALID".

* For a lineage check failure, the explicit KDC log is removed, and
  the status string is changed to "INVALID LINEAGE".

* For a user-to-user second ticket client mismatch, the explicit audit
  call is removed, and the log message does not include the second
  ticket client.

* e_data returned from the KDB check_policy_as() method will be
  included in the error for S4U2Self requests.

ticket: 8953 (new)